Method and apparatus for recovering heat from flue gases and for cleaning the same

ABSTRACT

The present invention relates to a method for recovering heat and separating substances from a flue gas streaming through a heat exchange apparatus comprising a casing with heat exchange elements inserted thereinto, a medium being provided for absorbing heat from the flue gas. According to the invention the medium that shall absorb heat from the flue gas is conducted through the heat exchange elements, the heat exchange elements and the connections to the casing for flue gas being so placed that the flue gas streams perpendicularly to the heat exchange elements, the flue gas being given a flow velocity cross the heat exchange elements of not more than 3 m/sec. Soot in the flue gas is collected on the heat exchange elements and adsorbs and/or dissolves gaseous substances such as sulphur dioxide. The invention also relates to an apparatus for carrying out the method.

This invention relates to a method and an apparatus for recovering heatand separating substances from a flue gas streaming through a heatexchange apparatus.

During combustion of oil and other fossil fuels, there are formed fluegases containing carbon oxides, sulphur oxides, nitrogen oxides, watervapour and soot particles. Other gaseous chemicals can be present in theflue gases from other types of fuels. These flue gases are normallyblown out into the air via a chimney. Flue gases from combustion of fuelthus contribute to atmospheric pollution and increased acidity in lakesand rivers. Actions are taken to reduce the atmospheric pollution, e.g.reduce the sulphur content in the fuel or clean the flue gases. Almostall sulphur in the fuel is oxidized to SO₂ during combustion and a smallpart is further oxidized to SO₃. When cooling the flue gases the SO₃reacts with water vapour and form sulphuric acid. The SO₂ can partly bedissolved in condensed moisture, specially in the presence of sulphuricacid, and forms sulphurous acid. Cooling of the flue gases leads tocorrosion of heat exchangers and chimneys thereby reducing thepossibility of recovering all the heat from the combustion of the fuel.Further, the condensed acid causes a conglomeration of oil and sootparticles present in the flue gases.

A known method is to cool the flue gases in a heat exchanger whereby thesulphuric acid is condensed and separated from the flue gases. Whencooling the flue gases, the heat can be recovered reducing the amount offuel to the boiler.

According to a known apparatus (see FIG. 1) the flue gases from acombustion room are introduced to a flue gas cooler commprising avertical heat exchanger provided with long straight tubes 10 of amaterial which shall resist corrosion. The flue gases stream inside thetubes at high velocity (more than 5 m/sec) partly so that the soot shallnot fasten on the inner tube wall and partly that a high heat transfershall be obtained. Between the tubes a liquid, for instance water, isheated by the flue gases inside the tubes 10.

The known apparatus, however, is impaired by a serious drawback. Thedesign of the apparatus and the high gas velocity makes the separationdegree for the sulphur low. Only 10-30% of the sulphur content in thefuel can be removed from the flue gases.

The present invention aims to recover energy from flue gases, to improvethe cleaning of the flue gases, especially reducing the sulphur contentin the same and to make a heat exchanger of corrosion resistant plasticmaterial. This purpose has been solved as is apparent from thedescription of the invention and the attached claims.

According to the invention there is provided a method for recoveringheat and separating substances from a flue gas streaming through a heatexchange apparatus comprising a casing with heat exchange elementsinserted thereinto, a medium being provided for absorbing heat from theflue gas, characterized in

that the medium that shall absorb heat from the flue gas is conductedthrough the heat exchange elements and has a temperature which is lowerthan the dew point of sulphuric acid,

that the heat exchange elements are so placed inside the casing of theheat exchanger and the connections to the casing for flue gas so madethat the flue gas streams essentially perpendicularly to thelongitudinal axis of the heat exchange elements,

that the flue gas is given a flow velocity cross the heat exchangeelements of not more than 3 118 m/sec.

that water vapour and sulphur trioxide in the flue gas form sulphuricacid when the flue gas is cooled, the sulphuric acid being condensedonto the outer walls of the heat exchange elements,

that soot in the flue gas is collected on the heat exchange elements andthis soot can adsorb and/or dissolve gaseous substances, such as sulphurdioxide, and

that the soot particles, sulphuric acid and the adsorbed or dissolvedSO₂ on the heat exchange elements are removed from the heat exchangeelements when the capcity of the soot layer to adsorb and/or dissolvesulphur dioxide has been reduced.

According to the invention there is also provided a heat exchangeapparatus for recovering heat and separating substances from a flue gasand suitable to use for the method according to claim 1, characterizedby the combination of the following features:

a casing provided with connections for a first incoming and outgoingmedium and comprising inlet and outlet modules, side modules and bottomand top modules;

heat exchange elements in the form of plastic tubes for a second mediuminside the casing, each of the tubes having helical form and two endsintended to be connected to means for incoming and outgoing,respectively, medium;

the helical, plastic tubes and the connections for the first mediumbeing so arranged that this medium streams essentially perpendicularlyto the longitudinal axis of the helical tubes;

the helical plastic tubes being so located in relation to each otherthat the distance between the centers of two adjacent tubes is less thanthe sum of the radii of the circles circumscribing the two tubes;

two adjacent helical tubes being locked in relation to each other byanother helical tube, one end of which being inserted into the commonspace of the two adjacent tubes;

the two ends of each plastic tube going through a plate of the bottommodule and/or the top module of the casing via holes in the plate, eachtube end being sealingly applied into the respective hole of the plate;

each side module comprising a metallic plate and an insulation coveredby a plastic layer, the plastic layer being in contact with the firstmedium.

The design, described above, of the heat exchanger will make the heattransfer and the cleaning of the flue gas more effective and theconstruction will be simple and costsaving.

According to a development of the invention the heat exchange apparatuscomprises plastic tubes, which are placed in rows after each otherinside the casing of the heat exchanger so that the plastic tubes fillout the space inside the casing both in its transversal direction andits longitudinal direction. By this arrangement the space inside thecasing is very effectively utilized, and therefore there will be roomfor many plastic tubes. Due to that fact the heat transfer will be stillmore improved.

According to a further development of the invention the plastic tubes inthe rows are alternately left-hand screwed and right-hand screwed. Dueto that fact the two different twisting tendencies arising in left-handscrewed and right-hand screwed plastic tubes can counteract and balanceeach other, whereby the geometrical form of the heat surface of theplastic tube can be maintained both in the cold and the warm state.

According to a further development of the invention the metallic plateof the module is of essentially the same size as the module and has aframe going round the plate on one side and is provided with theinsulation on its other side, which insulation is airtightly andliquidtightly enclosed by the plastic layer. That means that theinsulation protects the plastic tubes from being in contact with themetal wall which might have a high temperature above the melting pointof the plastic material. Hereby the casing will not damage the plastictubes.

According to a further development of the last-mentioned embodiment themodules are kept together by means of bolts going through the frame ofthe respective module, the sealing between the modules being broughtabout by means of a gasket made of a plastic material. Due to that factthe installation of the casing will be costsaving and the heat exchangeelements easily exchangeable.

The invention will in the following be described in more detail withreference to the accompanying drawings, in which

FIG. 1 shows a known heat exchanger for use in cooling and cleaning fluegases,

FIG. 2 shows a schematic view of an apparatus for cleaning flue gasesaccording to the invention comprising a heat exchanger in a flue gaschannel and an equipment for cleaning the heat exchanger,

FIG. 3 shows a transverse section through a schematically shown heatexchanger provided with heat exchange elements in the form of helicaltubes for use in the apparatus according to FIG. 2,

FIG. 4 shows a horizontal section of the heat exchanger according toFIG. 3, taken along the line IV--IV,

FIG. 5 shows a module according to the invention,

FIG. 6 shows a section through the module according to FIG. 5, takenalong the line IV--IV.

FIG. 7A shows a preferred embodiment of the tube arrangement in one rowof the heat exchanger and

FIG. 7B shows a tube arrangement in which the tubes are alternatelyleft-hand screwed and right hand screwed

FIG. 8A shows a horizontal section of FIG. 7A, taken along the lineVIII--VIII.

FIG. 8B shows a horizontal section of FIG. 7B taken along the lineVIII--VIII.

The apparatus shown in FIG. 1 has been described in the introductorypart of the description and does not need to be mentioned any more.

In FIG. 2 there is shown schematically an apparatus for cleaning thegases, comprising a heat exchanger and an equipment for cleaning theheat exchanger, which apparatus gives a very good separation of sulphurfrom the flue gases. Moreoever the heat exchanger is very effectiveregarding the recovery of heat from the flue gases.

The apparatus comprises a casing 20 provided with an inlet and an outletfor the flue gas from a not shown combustion station. Inside the casingthere are a plurality of heat exchange elements in the form of helicaltubes, in which the medium is conducted which is to absorb heat from theflue gas and cool down that medium.

The tubes are so located in the casing that the flue gases streamessentially perpendicularly to the longitudinal axis of each helicaltube, the heat in the flue gas is transferred to the medium in the tube,which medium has a temperature that is lower than the condensationtemperature of the sulphuric acid.

The material in each tube is preferably a plastic which is resistant tosulphuric acid. The plastic material makes the tubes cheap to produceand easy to bend into the helical form.

Another important feature in the invention is that the velocity of theflue gas in the heat exchanger is low, maximum 3 m/sec. This fact, incombination with the placing of the tubes and the material they are madeof gives the intended effect, i.e. a high separation of sulphur (ca.50%) from the flue gases. The reason is as follows.

The flue gas from oil and fossil fuels contains, as has been previouslymentioned, soot particles and among other chemicals sulphur oxides,which oxides comprise sulphur dioxide and a small percentage of sulphurtrioxide. When cooling down the flue gas in the heat exchanger,sulphuric acid formed of water vapour and sulphur trioxide is condensedonto the cold tube walls, which will be wetted by the sulphuric acid.The condensed acid causes a conglomeration of oil and soot particlesresulting in a cleaning effect of the flue gases flowing through theheat exchanger. As the velocity of the flue gases is low much soot willfasten onto the tube wall. The wetted soot can dissolve and/or adsorbgaseous chemicals such as SO₂. To this effect the emission of chemicalsand particles from the flue gases is reduced thus decreasing theatmospheric pollution.

An explanation to the reduction of gaseous chemicals in the flue gasesis partly that SO₂ is dissolved in condensed moisture, especially in thepresence of sulphuric acid, partly that soot is a carbon chemical whichhas a similar effect as active carbon when adsorbing chemicals from agas. However, an adsorbing mechanism is reduced after some time, as wellas at temperatures above 110° C. In order to re-establish the absorptioneffect, the soot layer on the tubes is removed and a new layer of sootwill gradually form again as described above. The removal of the sootlayer also reduces the fouling which has a negative effect on the heattransfer.

The removal of soot can be done by flushing with water. Thus, water froma pipe line 25 is sprayed via nozzles 26 over the heat exchanger tubes23. The soot with sulphuric acid and sulphur dioxide is removed to acollecting tank 27 under the heat exchanger. After the flushing the heatexchanger is ready for use again.

In FIG. 3 there is shown a section through a preferred heat exchangerfor use as a flue gas cooler in the apparatus according to FIG. 2. Theheat exchanger comprises a plurality of plastic tubes 30, in this caseten tubes placed side-by-side in a row, all of which have a helicalform. From each tube two tube ends 31, 32 extend, of which one 31 isconnected to a distribution means 33 for medium supply to the tube, thesecond tube end 32 being connected to a collecting means 34, to whichthe medium is conducted for further transport after having streamedthrough the tube 30. The tube ends 31, 32 pass through holes 35 made ina plate 36 of a top module of the casing 20 of the heat exchanger. Thus,the plastic tubes are connected to the means 33, 34 outside thecorrosive environment of the heat exchanger. This top plate 36 hasqualities allowing the plate to resist the corrosive environment, theworking pressure and the temperature in the heat exchanger. A suitablematerial for the top plate is a plastic material.

The tolerances between the plastic tube 30 and the hole 35 of the plate36 shall be so small that the tube must be forced through the plate by acertain force. The plate has a certain thickness motivated by thetemperature and the pressure inside the heat exchanger. The internalpressure of the plastic tubes together with the temperature of theinternal flow results in an increase of the outer diameter of theplastic tubes, whereby a sufficient sealing between the tube and theplate is achieved.

The top plate 36 forms one of the walls delimiting the casing of theheat exchanger. In FIG. 3 three further delimitation walls are shown,i.e. two side walls 37, 38 and a bottom plate 39 provided with adrainage outlet 40. Besides these two side walls 37, 38 there are aninlet module 41 and an outlet module 42 (see FIG. 4) in the heatexchanger, which modules are placed perpendicularly to the side walls37, 38. In the two modules 41, 42 there are connections 21, 22 forsupply and removal, respectively, of one of the media (in this case fluegas) participating in the heat exchange. This medium has in FIG. 4 beenindicated by the arrow 43. The plastic tubes and the connections for themedium passing the plastic tubes are so located that the medium streamsessentially perpendicularly to the helical plastic tubes.

As is shown in FIG. 4 the heat exchanger comprises a plurality osplastic tubes 30 placed in rows after each other inside the casing 20 ofthe heat exchanger so that the plastic tubes fill out the space insidethe casing both in its transversal direction and its longitudinaldirection.

Furthermore, the helical plastic tubes are so placed beside each otherthat they form a row over the essential width of the casing of the heatexchanger, the adjacent helical tubes being so placed that the distancebetween the centers of two adjacent helical tubes of the mentioned rowis less than the sum of the radii of the circles circumscribing the twotubes. Thus, the helical plastic tubes bent to a helical form restagainst each other. Furthermore, the plastic tubes in the rows or everyother tube in each row are preferably alternately left-hand andright-hand screwed, whereby the different twisting tendencies arising inthe plastic tubes can be counteracted. It should be noted that thetwisting tendency for right-hand screwed plastic tubes is of one type,while the twisting tendency for left-hand screwed plastic tubes is ofopposite type. Due to that fact these twisting tendencies balance eachother.

As has been previously mentioned the casing comprises two side walls 37,38, an inlet 41 and an outlet 42 module, and a bottom 39 and a top 36module. Each side wall is built up by one or several modules of the kindshown in FIGS. 5 and 6.

Each module comprises a metallic plate 50, which is made of steel and ofessentially the same size as the module. One side of the plate 50 isprovided with a frame 51 extending round the plate and applied at theperiphery of the plate. The other side of the plate 50 is provided withan insulation 52 which is made of a material resisting the aggressiveenvironment and which is covered by a plastic layer 53 made of e.g.polytetrafluoroethylene. The insulation 52 protects the plastic tubesfrom being in contact with the metal plate 50, whereby the plastic tubeswill not be damaged by the warm metallic plate. The layer 53 is kept inplace in a suitable way, e.g. by being fastened to the frame 51. Theplate 50 is intended to be applied so that the frame 51 is on theoutside of the heat exchanger, while the insulating material with theplastic layer is accordingly on the inside of the heat exchanger, wherethe corrosive environment is. The layer 53 is intended to protect theinsulation 52 and the steel plate 50 from corrosion. In the frame 51holes 54 are made at even intervals intended for bolts when fasteningseveral modules to each other or when fastening a module to the bottomplate or the top plate. For this last purpose also the top plate and thebottom plate are provided with a frame 55 and 56, respectively,extending round the plates (see FIG. 3).

The sealing between two modules and between a module and a topplate/bottom plate is preferably made by a gasket, which is appliedbetween the frames of the respective elements.

The advantage of the arrangement with module and top plate/bottom plateprovided with frames is that these elements are easy to assemble,disassemble or reassemble.

In FIGS. 7A and 8A there is shown a preferred embodiment of theinvention regarding the location of the different tubes in a row in theheat exchanger.

These figures show that two adjacent helical tubes 71, 72 engage eachother so that the distance between the centers 76, 77 of the tubes isless than the sum of the radii of the circles circumscribing the twotubes 71, 72. Each tube has two ends 78, 79, of which the end 78 isintended for supply of the medium going through the tube and the end 79for taking out the medium after having passed the tube. In thisconnection the end 79 of the tube 71 comprises a tube leg 71 U extendingfrom the bottom of the heat exchanger and through the top plate 80 ofthe heat exchanger to the outside of the same.

There is a common space between two adjacent tubes. Thus, in the FIGS. 7and 8 there is a common space 81 between the tubes 71 and 72. Throughthis common space and further through the top plate 80 (not shown), thetube leg 73 U of the tube 73 extends. Thus, the two adjacent tubes 71,72 are locked in relation to each other by the tube leg 73 U of the tube73. The next tube pair, the tubes 72, 73 is in the same way locked bythe tube leg 74 U of the nextcoming tube, i.e. tube 74, and so on. Byjoining two adjacent tubes in this way, these tubes constitute aself-supporting and a very stable unit which does not need any furthersupport or fastening devices.

I claim:
 1. Heat exchange apparatus for recovering heat and separatingsubstances from a flue gas having a casing (20) provided withconnections (21, 22) for a first incoming and outgoing medium (43) andcomprising inlet and outlet modules (41, 42), side modules (37, 38) andbottom and top modules (39, 36) and heat exchange elements in the formof plastic tubes (30) for a second medium located within said casing,each of said tubes having helical form and two ends (31, 32) intended tobe connected to means (33, 34) for incoming and outgoing, medium, thehelical, plastic tubes (30) and the connections (21, 22) for the firstmedium being so arranged that this medium streams essentiallyperpendicularly to the longitudinal axis of the tubes (30),characterized in thatthe helical plastic tubes (30) being so located inrelation to each other that the distance between the centres of twoadjacent tubes is less than the sum of the radii of the circlescircumscribing the two tubes, enclosing a common space (81); twoadjacent helical tubes (71, 72) being locked in relation to each otherby one end (73 U) of another helical tube (73) being inserted into thecommon space (81) of the two adjacent tubes (71, 72); the two ends (31,32) of each plastic tube (30) going through a plate of the top module(36) of the casing (20) via holes (35) in the plate, each tube end (31,32) being sealingly applied into the respective hole (35) of the plate;each side module comprising a metallic plate (50) and insulation (52)covered by a plastic layer (53), the plastic layer being in contact withthe first medium (43).
 2. Heat exchanger apparatus according to claim 1,characterized in that the plastic tubes (30) are placed in adjacent rowsinside the casing (20) of the heat exchanger so that the plastic tubesoccupy the space inside the casing both in its transversal direction andits longitudinal direction.
 3. Heat exchange apparatus according toclaim 2, characterized in that the plastic tubes (30) in each row arealternately left-hand screwed and right-hand screwed.
 4. Heat exchangeapparatus according to claim 1, characterized in that the insulation(52) comprises foam glass.
 5. Heat exchange apparatus according to claim1, characterized in that the plate of the top module (36) and the layer(53) are made of polytetrafluoroethylene.